Sulfuric acid reprocessor with continuous purge of second distillation vessel

ABSTRACT

An apparatus and method for reprocessing waste piranha containing contaminated H 2  SO 4  from, for example, a semiconductor processing operation is described. The apparatus and method include a first distillation flask which is maintained under a substantial vacuum. The first distillation flask includes a first column packed with a column packing material and an input pipe for refluxing to retard loss of H 2  SO 4  in the first distillation. The second distillation flask boils off substantially pure H 2  SO 4  and provides for a continuous automatic purge of the contaminants from the second distillation flask thus improving the purity of the H 2  SO 4 . The substantially pure H 2  SO 4  flows through a column which is coupled to a condenser which condenses substantially pure H 2  SO 4 .

BACKGROUND OF THE INVENTION

This is a continuation of application Ser. No. 07/261,916, filed10/24/88 now abandoned, which is a continuation in part of applicationSer. No. 07/231,849, filed Aug. 12, 1988, by the instant inventors andnow U.S. Pat. No. 4,980,032.

1. Field of the Invention

This invention relates to acid reprocessing and more particularly, to adouble distillation reprocessing of spent piranha acid to obtainsemiconductor grade sulfuric acid (H₂ SO₄).

2. Prior Art

Various methods for reprocessing acid are known, with doubledistillation being reasonably well known. Double distillation may beused when reprocessing spent piranha acid which is a combination of H₂SO₄ and an oxidant, such as H₂ O₂. In the semiconductor integratedcircuit manufacturing industry this combination (piranha acid) is usedto clean wafers and strip the photoresists.

The corrosive and toxic nature of the cleaning and stripping acids(e.g., H₂ SO₄) presents several problems. First, the problem of disposalis an economic problem as well as an environmental problem.Economically, it is costly to properly dispose of spent acid. Strictenvironmental control must be maintained. Even when environmentalregulations are observed and strictly complied with, there is always thepossibility of environmental pollution. Spilled acid travels quicklythrough the ground layer resulting in contamination of the aquifer andeventually reaches the ground water. In addition to the downwardmovement, a spill can creep through the ground sideways, thus creating agrowing contamination. In addition to the damaged environment, theliability cost associated with a hazardous waste clean-up is high, thusresulting in an even greater economic loss. Second, the piranha acid canonly be used once and when spent must be discarded, thereby requiringpurchase of additional acid. This is costly and restarts the disposalcycle.

Alternatively, piranha acid can be reprocessed, thus breaking thiseconomically and environmentally costly cycle. While acid reprocessingappears clearly to be a viable alternative, any acceptable process mustproduce H₂ SO₄ of sufficient purity to exceed the semiconductor industrystandards for sulfuric acid. The industry requires H₂ SO₄ to beextremely pure, with total metallic impurities being less than 500 ppb,typically much less. Particulate matter resulting from the cleansing ofwafers and stripping of photoresist must also be minimized. Preciseunderstanding of the source of particles in the liquid is as yetunrealized. However, small amounts of particulates can significantlyreduce the yield of semiconductor chips. For example, during thephotolithographic process, small particulates can adhere to the waferand result in the loss of a transistor or a conductor line, therebyresulting in a low yield, i.e. an increased loss of functional chips perwafer. Contaminates are even more disastrous when the process is usedfor VLSI fabrication.

It is believed that the high particulate count probably partiallyresults from the necessity of using high boiling temperatures in asecond stage of a double distillation process. Double distillation is aprocess used to purify acids and has been an established technology forapproximately 30 years. In the first distillation, low boilingcompounds, such as water, carbon dioxide and unreduced compounds aredistilled off from the acid. The acid, having a higher boiling point,remains in the distillation mixture and is transferred to the seconddistillation stage. The distillation mixture transferred to the secondstage contains the higher boiling point acid and other high boilingpoint compounds, such as heavy metal contamination and particulate.

Theoretically, these contaminants remain in the distillation vesselafter the high purity acid is distilled off. However, in practice, asmall portion of these contaminants and particulates are distilled off.The particulates escape the liquid phase of the mixture and are carriedover into the distillation column in the prior art second stagedistillation along with the gaseous H₂ SO₄.

Accordingly, considerable effort has been directed to devising a processthat reduces this carry over improving the purity of H₂ SO₄. Inparticular, much effort has been directed at evolving a process thatdecreases the particulate matter and other contaminants.

SUMMARY OF THE PRESENT INVENTION

An acid reprocessor apparatus and process for reprocessing piranha(waste) acid from a semiconductor wafer cleaning and etching process isdescribed.

The waste acid is processed through double distillation, therebyincreasing the purity of the product H₂ SO₄ to meet semiconductor gradestandards. Further, the double distillation process maximizes the purityof the product. Reducing the pressure advantageously employs aproportional relationship between pressure and temperature. Since theboiling point of a material is reduced as the pressure is decreased,safer temperatures may be maintained. In the first distillation, adistillation flask means is operated at the boiling point of relativelydilute (typically 80-95%) H₂ SO₄. However, to assure that H₂ SO₄ is notdistilled at this first step, dilute acid is trickled through the columnof the distillation flask means thus removing gaseous H₂ SO₄ from thedistillation vapor.

Once the lower boiling compounds are removed from the distillationmixture and the acid is at the proper concentration, the mixture istransferred to a second distillation flask means. At this point theenriched feed is heated to reach the boiling point of H₂ SO₄, which hasbeen reduced by decreasing the pressure to approximately 5 Torr in thesecond distillation flask means. Decreasing the pressure reduces theboiling point of the H₂ SO₄. The reduction of the temperature causes thechemical activity differential between the product H₂ SO₄ and the higherboiling metallic impurities to increase (i.e. the difference between thechemical activity of H₂ SO₄ and the chemical activity of higher boilingmetallic impurities increases), thus ensuring a purer product because ofreduced level of these impurities.

As the gaseous H₂ SO₄ exits the second distillation flask means, a smallamount of the second distillation flask contents is continuouslytransferred into the sludge reservoir and periodically removed from thesludge reservoir to a waste collection tank. The H₂ SO₄ vapor from thesecond distillation flask means is liquified through a condenser. Aself-contained, recycled coolant system is maintained through thecondenser. The reprocessing system provides for further recycling if thequality assurance system indicates that the product is not within therequired specifications. In addition to the safer conditions of loweroperating temperatures, the system is maintained with numeroustemperature sensors and liquid level monitors which will indicate anysafety threatening problems. Multiple alarms are triggered when suchproblems arise as well as when product readings are out ofspecification.

The process and apparatus of the invention yields more purified H₂ SO₄(relative to the prior art) because the terminal velocity of theparticulate contaminants are decreased by the vacuum in the seconddistillation stage. Moreover, the use of a vacuum in the second stage ofdistillation permits the reduction of the temperatures required tovaporize the H₂ SO₄ ; therefore, less expensive equipment may beutilized in the apparatus of the invention relative to prior artapparati. The lower temperatures also reduce the wear and disintegrationon the equipment and, therefore, improve the reliability of theequipment. Furthermore, the continuous (and controlled) removal of asmall amount of the contents of the second distillation flask improvesthe purity of the H₂ SO₄.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of the piranha acid reprocessing system.

FIG. 2 is a detailed schematic representation of the preliminarypreparation apparatus prior to distillation.

FIG. 3 is a detailed schematic representation of the double distillationapparatus.

FIG. 4 is a detailed schematic representation of the apparatus employedfor contaminant removal.

FIG. 5 is a detailed schematic representation of the coolant system inthe apparatus.

FIG. 6 is a detailed schematic representation of the vacuum pump system.

FIG. 7 is a block diagram of the product removal and quality assuranceloop.

FIG. 8 is a chart showing the relationship between the vacuum pressurein the distillation system at various temperatures and concentrations ofthe dilute acid reflux in the column 255.

FIG. 9 is a schematic representation of the second distillation flask D2shown in FIG. 3.

FIG. 10 is a chart showing the results of concentration of distillate(C_(D)) versus time (t) when the partition co-efficient is 5000 for apurged and a non-purged distilling system.

FIG. 11 is a chart showing the results of concentration of distillate(C_(D)) versus time (t) when the partition co-efficient (p.c.) is 1000.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

A piranha acid double distillation reprocessing method and apparatuswhich provides for semiconductor grade H₂ SO₄ at lowered temperaturesand decreased pressures is described. In the following description,numerous specific details are set forth such as specific temperatures,pressures, materials, etc., in order to provide a thorough understandingof the present invention. It will be obvious, however, to one skilled inthe art that the present invention may be practiced without thesespecific details. In other instances, well known techniques and deviceshave not been described in detail in order not to unnecessarily obscurethe present invention.

In FIG. 1, a piranha reprocessing system is schematically represented.The spent piranha acid ("feed") is introduced to the system via theinput line 1. The feed enters the preliminary preparation apparatus 2("input section") where the temperature is stabilized. Level controllersand temperatures sensors are also provided in the input section. Once aprespecified temperature and level in the receiving flask is reached,the feed is drained into the product distillation system 3 shown inFIG. 1. It is during this phase of reprocessing that the pressure isdecreased and the temperature is further increased. Prior to thejunction at 3b as shown in FIG. 1, the operation is conducted atatmospheric pressure.

From the product distillation system 3 in FIG. 1, purified H₂ SO₄("product") is removed to the receiving tank 8 or recycled at junction11 (by opening valve V₈) depending on the analysis from the qualityassurance system loop 7. Also from the product distillation system 3,gaseous water and liquid waste acid and heavy metal contaminants andparticulates ("sludge") are removed into the waste removal system 4. Thesludge is drained from the sludge reservoir in the removal system 4 to awaste collection tank 9 ("T-3").

The coolant system 5 is a self contained unit which is used to vary thetemperature of the coolant in the condenser of the product distillationsystem 3. A water supply 10 cools the heat exchanger of the wasteremoval system 4.

The pump system 6 provides a vacuum in certain structures in productdistillation system 3 and waste removal system 4. While the pump systemis also connected to the preliminary preparation system 2, the connectedlines are activated only to purge the system, since as previouslyindicated the preliminary preparation system 2 is at atmosphericpressure (while structures in the product distillation system 3 are atsubstantially vacuum pressures).

To completely describe the process and apparatus of the presentinvention, it is necessary to describe in detail the requisite apparatusand its operation. Therefore, Part I is a detailed description of theapparatus and refers generally to FIGS. 2-7. Part II is a detaileddescription of the operation of the apparatus and covers the followingoperations: start-up sequence; idling sequence; normal operations;auto-recycling sequence; quality assurance sequence and safety alarmprovisions.

PART I General Features of the Apparatus

Referring to FIGS. 2-7, the valves V1-V22 are selectively opened andclosed to control the direction of the flow and the ultimate destinationof gases and fluids. In addition to flow regulation via opening andclosing of the valves V1-V22, the flow rate may be monitored by a flowmeasurement device such as the flow measurement device attached to line108 in FIG. 4. The valves V1-V22 may be controlled manually by a seriesof separate switches or combination of switches that the user may use toshut down the apparatus systematically.

Liquid level monitors and temperature sensors may be integrated into asystem alarm to indicate unusual or dangeous conditions. The temperaturesensors may be a thermocouple with a preset temperature and may beinterfaced with a shut down mechanism on a heating system such that oncethe predetermined temperature is reached, the heating system is shutdown. Moreover, the temperature sensors may be an integral part ofheating systems which automatically maintain a temperature which is setby the user of the heating systems; such systems are commerciallyavailable. The level sensors used with the apparatus of the inventionare conventional, commercially available level sensors (e.g.acoustical/ultrasound or optical liquid level sensors). The temperaturemonitors and liquid level monitors allows the user to continuously checkif the system is within the desired ranges.

Referring to FIGS. 2, 4, 5 and 6, several pumps P1, P2, P3, P4 and P5are shown. The pumps P1-P4 as shown in FIGS. 2, 4 and 5 are simple feedpumps. For example, pump P1 as indicated in FIG. 2 may be used to drivethe feed forward to the input line 1. Pump P2 as shown in FIG. 4 is usedto drive liquid H₂ SO₄ from the output of the heat exchanger HE1 to thetop of the absorption column AD1. Pump P3 as indicated in FIG. 4 is usedto drive deposited waste acid into a waste collection tank. Pump P4 maybe a single stage rotary pump as shown in FIG. 5 which may be employedto circulate coolant through a self-contained system. These pumps areconventional, commercially available items.

The fifth pump, P5, is shown in FIG. 6 and is required to evacuate theapparatus to pressure levels of 5-10 Torr. This may be accomplished byusing a simple conventional vacuum pump, such as an oil pump as in thepresent embodiment.

The nodes (e.g. Node 1) merely identify the points of interconnectionbetween the various figures and are shown for the convenience of thereader. For example, it will be understood that Node 1 of FIG. 2 isconnected to Node 1 of FIG. 3 and therefore, spent piranha from flask F1of FIG. 2 is conveyed through a pipe means ("line") 102 to the firstdistillation flask means D1. It will be understood that liquids aretransferred from one vessel to the next vessel by gravity feed wherepumps are not shown. For example, gravity feed will provide themechanism for transfer from vessel D1 to vessel D2 and for the transferfrom sludge column 255 to purge column F4.

A. Preliminary Preparation System Apparatus (Input Section)

FIG. 2 shows a detailed representation of the input system 2 of FIG. 1.The input line 1 may be a pipe means ("line") which can be constructedfrom noncorrodible material such as Teflon® or Kynar® or any othersimilar materials. It will be appreciated that the other lines shown inFIGS. 2-7 are also pipe means which may be constructed from Teflon® orconventional borosilicate glass (e.g., Pyrex®); these pipe means areused to couple the various components (flasks, distillers, etc.). Theinitial filter FI4, with a pore size of 100 microns in the preferredembodiment, eliminates the majority of particles in the feed. Of course,it cannot eliminate particles created from further processing, such asthe processing in the first distillation stage.

A simple pump P1 is positioned between valve V9 and the primaryreceiving flask F1 (input flask means). The primary receiving flask F1may be constructed from any heat and chemical resisting material;however, borosilicate glass (e.g. Pyrex® glass) is used in the preferredembodiment. Advantageously, flask F1 is coupled to a liquid level sensorand to a temperature sensor. The spent piranha is fed into the primaryflask F1 from the line 101 through valve V9. The piranha in the primaryflask F1 is heated by a heating means H1, such as a heating mantle inthe present embodiment. The temperature of the piranha in flask F1 istypically maintained at approximately 175° C. The liquid level andtemperature is monitored and regulated from the level sensor andtemperature sensor which are attached to flask F1.

B. Product Distillation System

The apparatus shown in FIG. 3 comprises the product distillation system3 of FIG. 1. The feed exiting from the primary flask F1 through valve V1is transferred to a distillation vessel D1 (first distillation means)via line 102 and is heated in the vessel D1 by a heating means H2, suchas a heating mantle as in the present embodiment. The heating means H2is typically an automatically regulated heating device, the temperatureof which is controlled based upon the temperature setting selected bythe user and upon the output from a temperature sensor. Such heatingmeans are commercially available. The heating means H2 periodicallyheats the vessel D1 to the requisite temperature (e.g. a temperature inthe range of 200° F. to 300° F.) as specified by temperature sensor andthe user selected temperature setting. The top of the vessel includes agaseous output and is capped with a packed distillation column DC1; thatis, the distillation column is packed with a column packing means PM1.Although Rashig rings are used as the packing material PM1 in the columnin the preferred embodiment, other suitable column packings such asLessing rings or glass beads may also be used. An input pipe 23a, forreflux liquid (e.g. deionized water), is located near the top of thedistillation column DC1 above the column packing material but below amist eliminator M1. Line 103 is coupled to mist eliminator means M1.Reflux liquid is discharged from the input pipe 23a and trickles overand through the packed column DC1. A source of de-ionized water ismaintained under a vacuum and is coupled to valves V19a and V19b; thede-ionized water is supplied to input pipe 23a through valves 19a and19b. The vapor outlet, located at the upper-most portion of the columnDC1 along with a mist eliminator M1, is coupled to the waste disposalsystem 4 of FIG. 4 through line 103. The mist eliminator M1 isessentially a deatomizer removing any particles or condensation from thevapor, thus ensuring that only the desired vapor from the distillationmixture is inputted to the waste output system 4. Mist eliminator meansM1 may be comprised of glass wool or other suitable means for removingparticles and condensed water droplets from the gaseous water leavingD1. The line SL1 of the distillation vessel D1 provides a means fortransferring the distillation mixture containing the partially purifiedH₂ SO₄ to a second distillation means vessel D2 through valve V2 andline 107, thus providing careful regulation of the transfer between thefirst and second distillation vessels. The vessel D1 and the column DC1may be constructed from borosilicate glass such as Pyrex®.

The second distillation vessel D2 (second distillation means) is similarto the vessel D1. D2 is also seated in a heating means, such as aheating mantle H2, which heats the distillation mixture to a temperaturehigher than that of vessel D1. An attached temperature sensor is used tocontrol and monitor the temperature within the distillation vessel D2.The vessel D2 includes an input which is coupled to line 107. Thedistillation vessel D2 is capped by a distillation column DC2. Unlikethe distillation column of D1, there is no deionized water trickle inputpipe at the top of the distillation column for D2. However, there is avapor outlet which is coupled through a mist eliminator M2 to thetubular shell of the condenser C1. An outlet located near the bottom ofdistillation vessel D2 provides for the release of the remaining sludgemixture in vessel D2 which flows via line 105 through the open valve V3to the waste disposal system of FIG. 4. In an alternative embodiment,valve V3 may be periodically opened and closed.

The product (H₂ SO₄) in its gaseous state travels through line 104 andenters the tubular shell (e.g. glass jacket) of condenser C1 where it iscooled to its liquid state by the coolant flowing through the coil 275of the condenser C1 and then it collects in the lower portion ofcondenser C1 until it overflows into the pre-product vessel flask F2.The product then proceeds through check valve CV1, valve V4 and into thepre-product vessel F2 shown in FIG. 7. The vessel D2, the column DC2 andthe condenser C1 may be constructed from borosilicate glass, such asPyrex®.

C. Waste Disposal System

The waste disposal system 4 is shown in FIG. 4. This system is comprisedof a purge column F4 and a sludge column 255 which includes a heatexchanger means HE1. The sludge column 255 receives inputs from thedistillation column DC1 through line 103, the condenser C1 through line110, and the distillation vessel D2 through line 105. The heat exchangermeans HE1 is positioned in such a manner as to allow the heat exchangerHE1 to fill to a level controlled by the filling of distillation flaskD2 and valve V3. The level of sludge may be carefully monitored by aliquid level sensor attached to sludge column 255, and that level iscontinuously added to through open valve V3 by continuously purging(removing) the remaining fluid in the vessel D2. The sludge column 255also includes an absorption column AD1. Although, the packing materialPM2 of the column AD1 is Rashig rings in the preferred embodiment, anyother suitable packing material such as Lessing rings or glass beads maybe used.

The sludge exiting line 105 into the heat exchangers HE1 passes over thecoils 256 of the heat exchanger HE1 and is cooled by the cool waterflowing through the coil 256. The coil 256 of the heat exchanger HE1 isfilled with cooling water, which flows through a closed loop (via lines115a and 115b) which is temperature controlled through the exchanger HE2shown in FIG. 5. The sludge, which is under vacuum in the sludge columnduring normal distillation processing, will collect within the tubularshell of HE1 until drained periodically into the purge column F4, whichis also under vacuum during normal distillation processing. The purgecolumn F4 may be drained while continuing an ongoing distillation (undervacuum in vessels D1, D2, C1 and the sludge column 255) by closing valve11 and allowing the purge column F4 to come to atmospheric pressure (byclosing valve V14 and opening valve V13) and then by pumping the sludgeout of purge column F4 by pump P3 through momentarily opened valves V12and CV4 and into a waste collection tank. The purge column F4 may befilled from the sludge column 255 while continuing an ongoingdistillation by opening valve V11 while the purge column F4 is undervacuum (the purge column F4 is kept under vacuum by keeping valve 14open while valves V13, V15 and V12 are closed).

A reflux liquid inlet 23b is positioned above the absorption column AD1packing. A small portion of the output of the heat exchanger HE1(typically a liquid acid waste) is tapped off at valve V11 and is pumpedvia pump P2 through line 108 to the top of the absorption column AD1where the liquid acid waste then trickles down through the packingmaterial forcing any vaporous materials back through the heat exchangerHE1.

D. Coolant System

The cooling system 5 shown in FIG. 5 is comprised of two interconnectedclosed-loop systems. A pump P4, which may be, for example, a singlestage rotary pump in the preferred embodiment, circulates the coolantwhich is typically an oil (e.g., Dowtherm) in lines 114a and 114b andthrough the system which includes HE2 and C1. Temperature increases areprovided by a variable heater H5 (variable heater H5 may be anyconventional variably controlled heating means such as a variablycontrolled heating jacket which surrounds line 114b) with thetemperature being monitored at an output of heater H5 by temperaturesensor T13. The oil coolant is circulated through the coil 275 of thecondenser C1 which is disposed within the tubular shell of the condenserC1. The coolant is also circulated through the jacket (tubular shell) ofheat exchanger HE2. Both the input and the output to the coil 275 ofcondenser C1 are monitored with temperature sensors, includingtemperature sensor T13. Cooling water is circulated through the secondsystem comprised of the heat exchanger HE1 and the heat exchanger HE2and lines 115a and 115b. The cooling water circulates through the lines115a and 115b and through the coils disposed within heat exchangers HE1and HE2. Input valve IV1 and control valves V20a and V20b provide thecooling water to HE1 and to HE2 from an outside source of water,typically at room temperature. Temperature and flow sensors may be usedto monitor temperature and flow parameters. The temperature of the watercoolant may be altered at heat exchanger HE2 by varying the temperatureof the oil coolant in the first system.

E. Vacuum Pump System

The pump system 6 is described in conjunction with FIG. 6. To attainvacuums of 5-10 Torr, the vacuum pump P5 may be an oil pump. The pump P5is attached to an external, conventional gas scrubber. To protect theinput of the vacuum pump P5, a conventional vapor trap VT3 is installedto trap any gaseous vapors that may come through the lines. The pressureis monitored by a conventional pressure control sensor PC1. Nitrogen gas(N₂) may be used to compensate the pressure reduction produced by thevacuum pump P5 by bleeding N₂ into the input of pump P5 through valveVN₂.

F. Quality Assurance System

FIG. 7 shows the product removal and quality assurance system 7. Theproduct exiting from the condenser C1 is directed through line 106 andthe valves CV1 and V4 into the pre-product flask F2. The product may bedriven by gravity to the flask F2. From an output line 111 of flask F2 aproduct sample is analyzed by a conventional particle counter BB1, by aconventional ion detector BB2, and a conventional density monitor BB3measuring the particle count and metal contaminants remaining after thereprocessing cycle. If the analysis indicates levels that are not withindesired specifications, the product is shunted through valve V8 into theinput line 1 where it is recycled for reprocessing. If the analysisindicates levels that are within the desired specifications, then valvesV5, V17, CV2 and V7 are opened (with valves V18, V4 and V8 closed) andthe product drains from flask F2 into a product collection tank 8. ValveV17 is coupled to a venturi vacuum pump which is coupled to aconventional gas scrubber; opening valve V17 releases any pressure inflask F2, and allows a rough vacuum to be pulled using vapor trap VT1before opening V18 to reconnect to the vacuum system.

PART II The Operation A. The Start-Up

When the system is initially set up or has been completely purged andcooled, an allotment of approximately several (e.g. 4) hours should beused to bring the system to operating temperature and pressure. The slowstart-up is designed to minimize thermal stress to the system. Minimalstress lengthens the lifetime of the apparatus' components, which isdesired from both an economic and safety standpoint. Components weakenedby stress will be more likely to initiate a safety hazard ormalfunctions.

Referring generally to FIGS. 1-7, the start-up operation proceeds asfollows:

The front end of the process, i.e. the preliminary processing up tovalve V1 is conducted at standard atmospheric pressure (760 Torr). Theinput line 1 is coupled to the pump P1 which pulls the feed from theline 101 through the input filter FI4 and input valve V9. A desiredamount of feed is pumped into the flask F1 via line 101. After heatingthe feed by a heating means H1 to a temperature in the range of 175° C.(347° F.), the feed is drained through line 102 and valve V1 into thefirst distillation flask D1.

Once the feed is in distillation flask D1, valve V1 is closed and theheating means H2 slowly raises the temperature of the feed to anoperating temperature of greater than 175° C. (347° F.). During theheating process at distillation means D1, the vacuum pump system 6(vacuum generation means) decreases the pressure by means of the vacuumpump P5. Concurrent with this latter operation, i.e., increasedtemperature and decreased pressure, the oil coolant system 5 is startedup and raised to an operating temperature of approximately 105° C. (221°F.).

The vacuum pump P5 pulls a vacuum in vessels D1 and D2 through thecondenser C1 and the sludge column 255. The sludge column 255 is coupledto the vacuum pump system at node 2 as shown in FIGS. 4, 5 and 6; thepurge column F4 is coupled to the vacuum pump system through valve V14at node 2 as shown in FIGS. 4 and 6. The pre-product flask F2 is coupledto the vacuum pump system through valve V18 at node 3 as shown in FIGS.6 and 7. It will be appreciated that during the generation of a vacuumin vessels D1 and D2, and in condenser C1 and sludge column 255, valvesV1, V4 and CV1 are closed. After filling the flask D1 with spent piranhaacid, valve V1 is closed to allow the generation of a vacuum. Similarly,valves V11, V12, V17, V5 (and CV2), V13 and V15 will normally be closedwhile generating a vacuum in vessels D1 and D2. The vacuum in D2 isgenerated through line 104, condenser C1, line 110, and the sludgecolumn 255, and the vacuum in D1 is generated through line 103 and thesludge column 255. After the vacuum is generated to the operating levelsspecified below, the apparatus may be used in the standard mode ofoperation.

B. The Standard Mode of Operation

Referring to FIGS. 1-7, the standard mode of operation proceeds asfollows:

The feed from input line 1 proceeds as previously described in Part II,(Start-Up) i.e. the feed is pumped into the flask F1, through the inputfilter FI4 and input valve V9. The input filter FI4 eliminates most ofparticulate matter greater than 100 microns prior to the actualdistillation process.

The feed reservoir in flask F1 is maintained at standard atmosphericpressure and heated to a temperature of about 175° C. (347° F.). Thetemperature is maintained within this range by continuous monitoring oftemperature sensor T1 and controlling a heating means H1 (e.g. a heatingmantle) which surrounds the flask F1. Through valve V1 via line 102, thefeed is collected in the first distillation flask D1. After the flask D1is filled, the valve V1 is closed to allow for the vacuum to bestabilized in flask D1. In the first distillation flask D1, thetemperature range is maintained between 149° C.-204° C. (300° F.-400°F.), and the operating pressure is decreased to a range of 5-25 Torr.

In the first distillation flask D1, the lower boiling point compoundssuch as water and unreduced compounds are separated from the acid. Thedistillation column DC1 attached to the flask D1 is packed with a columnpacking means PM1 (e.g. glass rings or beads, such as Rashig rings). Asthe mixture in flask D1 is heated, the water is boiled off, rising intothe packed column of D1 and exiting at the uppermost outlet into line103 after passing through a mist eliminator M1. However, as the H₂ SO₄and H₂ O rise into the column it is mixed with a trickle of de-ionizedwater provided through an input pipe 23a at the top of the packed columnDC1, thereby causing the H₂ SO₄ to be recondensed into the distillationmixture within the flask D1. After the acid has reached the desired(e.g. 97%) concentration the valve V2 is opened and distillation mixtureis drained through line 107 and valve V2 into the larger seconddistillation flask D2 where the next step of the process commences.After filling flask D2, the valve V2 is closed. The second distillationflask D2 is maintained at a higher temperature range of approximately190° C.-218° C. (375° F.-425° F.) at a lower pressure of approximately 5Torr. In addition to the heating means H3 which surrounds the flask D2,several smaller heating means H4a-c are positioned within thedistillation flask D2. To provide for more efficient agitation of thedistillation mixture, a stirrer ST1 is also included in the distillationflask D2.

In the second distillation, high purity H₂ SO₄ is distilled. The higherboiling compounds (e.g., heavy metals) are retained in the bottom of thedistillation flask D2.

Decreasing the pressure, particularly in flask D2, decreases the boilingpoint of the H₂ SO₄, thus allowing the system to be operated at a lowertemperature. Lowering the pressure also lowers the density of theproduct gas. Decreasing the temperature causes the differential betweenchemical activities of the H₂ SO₄ and of the heavier sludge to increase.As the difference of the two activities is increased the likelihood ofincreased product purity is also increased. Since the density of the gasand therefore the terminal velocity of the particles (e.g. particulatecontaminants) is decreased, the ability of the particles to escape theliquid phase is reduced. Therefore, the particles remain within thedistillation mixture within the flask D2. In addition to the particlesin the distillation mixture, the sludge contains (metals) compounds thatboil at a higher temperature. These metal compounds have a greaterdependency on temperature changes. Thus as the pressure is decreased,the boiling points of the sludge and the H₂ SO₄ change at differentrates. The different dependencies increase the ability to separate thedesired H₂ SO₄ from the metal compound contaminants.

As the H₂ SO₄ in flask D2 is converted into the gaseous (g) form, itrises through the distillation column DC2 which caps the distillationflask D2 and rises through the mist eliminator M2. The H₂ SO₄ (g) flowsout of the distillation column DC2 via line 104 and into the primarycondenser C1 at which point the gaseous H₂ SO₄ condenses into highlypure liquid H₂ SO₄ which flows into a lower reservoir of the primarycondenser C1. However, the temperature of the product is still elevatedbeyond that of room temperature and the product remains in the condenserC1 until the product is drained from the condenser C1 into the flask F2through line 106. The condenser C1 is drained usually only when theflask F2 is kept under a vacuum. Thus, if any product has previouslybeen drained into flask F2, that product in F2 will be under vacuum(valve 18 open while valves V17, CV2 and V5 are closed). The condenserC1 is drained (by gravity feed) when valve V4 is opened after a vacuumhas been established in flask F2; after filling flask F2 to the desiredlevel, valves V4 and CV1 are closed. The product may be drained bygravity feed from flask F2 while continuing an ongoing distillation byclosing V18 (and assuring that valve V4 is closed) and by opening valveV17 to bring the pressure in flask F2 back to atmospheric pressure.After draining the desired amount of product from flask F2 through line111 and valves V5 and CV2, the flask F2 is again depressurized toproduce a vacuum in flask F2 (by closing valves V17, CV2 and V5 and byopening valve 18).

The distillation mixture remaining in vessel D2 is coupled through line105 and valve V3 to an input of the sludge column 255. Valve V3 is openand the sludge and other materials (fluid) remaining in D2 arecontinuously purged from D2 and flow through line 105 entering the heatexchanger means HE1 at the lower portion of the sludge column 255. Thecontinuous removal of sludges allows lower concentrations of sludge andother materials to remain in the vessel D2, therefore improving thepurity of the product. As shown in FIG. 9, the valve V3 is set (opened)so as to regulate the flow ("q") though line 105 from the vessel D2 tothe sludge column 255. Flow meters, such as flow meters 201 and 202, maybe advantageously used to adjust valve V3 to regulate the flow thoughline 105 relative to the flow in line 107. It will be appreciated thatregulating the flow through line 105 will also regulate the continuousdraining of vessel D2 during normal distillation operations. Rather thanuse a flow meter, one may determine the flow rate volumetrically byfilling D2 with a known quantity (volume) of fluid and then bydetermining the time it takes to drain D2 (knowing the volume and time,one can calculate the flow rate).

Referring to FIG. 9, the vessel D2 is schematically represented. Forillustration assume the following: C_(R) =C_(B) where C_(R) equals theconcentration of contaminants in ppm of the mixture (_(R)) in the vesselD2 and C_(B) equals concentration of contaminants in the waste (_(B)).Further assume that q_(F) =q_(D) +q_(B) (what goes in must come out),where q_(F) is the volume flow rate in liters per minute (1/min) of thefeed (_(F)) from line 107; q_(D) is the volume flow rate in 1/min of theproduct (_(D)) through line 104 and q_(B) is the volume flow rate in1/min of the waste (_(B)) through line 105, and assume that the ratio ofthe concentration of a particular contaminant in the product (C_(D)) isproportional to its concentration in the vessel D2 (C_(B)) or bC_(D)=C_(B) where b is the partition coefficient constant. The larger b is,the better the results.

The volume of liquid purged from the vessel D2 (q_(b)) is arbitrary andmay be expressed as a fraction, (a), of the feed stream, i.e. q_(B) =aq_(F). Therefore,

    q.sub.F =q.sub.B =q.sub.D+ q.sub.B

which may be written as ##EQU1##

For example, in the case of no purging that is, when a=0, q_(B) =0 andq_(D) =q_(F). However, a purged system will allow for lowerconcentration of contaminants in the product (distillate) for a longerperiod of time, even if the purged system has a lower partitioncoefficient (b).

The accumulation of the contaminants may be defined as ##EQU2## whereV_(R) is the volume of fluid in D2, t is time, and dC_(B) /dt is thederivative of C_(B) with respect to time; this definition results fromthe fact that the accumulation of the contaminants in vessel D2 equalsthe amount of contaminants into vessel D2 minus the amount out of D2.The immediately preceding equation (1), by substituting for q_(F) andq_(B) and by rearranging, may be written as ##EQU3## By rearranging theimmediately preceding equation after letting

    k=(1-a+ab)/b

that equation may be rewritten as the following differential equation##EQU4## After integrating the left side of this differential equationfrom C_(o) to C_(B) and integrating the right side from O to t, andafter solving for C_(B), one obtains the following equation: ##EQU5##where exp=e=2.71 . . . , and where C_(B) is, of course, theconcentration at time t of the contaminants (or a particularcontaminant).

Using the above equation (IV) in the limiting case as when there is nopurge, that is a=0 and hence k=1/b, then that equation (IV) becomes:##EQU6## Solving equation (V) for the non-purged system at t=∞ showsthat C_(B) =bC_(F). Remembering that C_(B) =bC_(D), then C_(D) =C_(F),in other words what goes into the vessel D2, comes out of the vessel D2.

On the other hand assume that a=0.05 (i.e. a 5% purge). The results ofthe equation IV with a 5% purge over periods of time (t) where b=5000are illustrated in FIG. 10. Also shown in FIG. 10 are the results ofthat equation with no purge over the same periods of time where b=5000,a reasonable number for the partition coefficient (p.c.).

Viewing the chart in FIG. 10 illustrates the advantage of a purgedsystem. FIG. 10 assumes a feed concentration of the contaminant (C_(F))to be 100 ppb and an initial concentration (C_(o)) of the contaminant inthe distillate to be 1 ppb. If 10 ppb is considered unacceptable (i.e.not within specification), then the non-purged system goes out ofspecification after approximately ten (10) days. On the other hand, thepurged system (with a=0.05) never goes out of specification since att=∞, C_(D) =C_(F) /bk so C_(D) =3.98×10⁻¹ ppb, while for the non-purgedsystem at t=∞, C_(D) =C_(F) =100 ppb. It will be appreciated that if thepurge rate of D2 is 5% (a=5%), then q_(B) should be set to be 5% ofq_(F). In practice, the purge rate will range typically from 1% to 5%,but users may purge D2 at rates as high as 50% to improve the purity ofthe distillate.

Even if one assumes an inferior partition coefficient (b) as shown inFIG. 11 where b=1000, the purged system is better than a non-purgedsystem. If assuming t-∞, then C_(D) =1.96 ppb.

However, if rigid specifications are not required, an alternativeembodiment may be practiced that allows the operator to periodicallypurge D2 through the valve V3, thus allowing the sludge and othermaterials to flow through the line 105 into the heat exchange means HE1.

The sludge column 255 also has an input from the condenser C1 via line110 and has an input from the gaseous output of vessel D1 via line 103.Line 103 provides water and other low boiling compounds from vessel D1into the sludge column 255; the water from vessel D1 tends to dilute thesludge from vessel D2.

The diluted sludge rests in the heat exchanger means HE1 in the bottomof the sludge column until the sludge column 255 is drained by openingvalve V11. The diluted sludge, which is typically a weak acid, isrecirculated through the sludge column 255 by pumping (by pump P2) aslow trickle of the diluted sludge through line 108 (and open valveV10). The diluted sludge trickles from the input pipe 23b and throughthe packing material PM2 of the absorption column AD1 and back into theheat exchanger HE1. The trickling of diluted sludge through the sludgecolumn 255 tends to absorb any vapor and keep it within the column 255.The coil 256 of the heat exchanger HE1 tends to cool the sludge mixture(while heating the water coolant) and thereby heating cooling waterwhich flows through the tubular shell of the heat exchanger HE2.

During an on-going distillation, the sludge column 255 is drained byfilling (via gravity feed) the purge column F4 while the column F4 iskept under vacuum. Column F4 is kept under vacuum during normaldistillation operations by keeping valve V14 open while valves V12, V13and V15 are closed. When valve V11 is opened while column F4 is undervacuum, the diluted sludge from the sludge column 255 flows (by gravity)through line 109 into the column F4. Typically, column F4 is positionedrelative to column 255 so that some diluted sludge remains in the column255 to cover the coil 256. After draining a desired amount of dilutedsludge into the purge column F4, valve V11 is closed.

During an on-going distillation, the purge column F4 may be drained byopening valves V12, V13, and V15 while closing valve 14 (valve V11 willof course also be closed). The diluted sludge from F4 is pumped by pumpP3 to a waste collection tank.

The dilute acid (diluted sludge) which is trickled (refluxed) throughthe sludge column 255 will have an impact on the vacuum pressure in thedistillation system, including the vacuum pressure in the vessel D2.Specifically, the amount and temperature of the trickle (reflux) throughcolumn 255 will set a minimum limit on the vacuum pressure; up to acertain point at a given temperature of the reflux, a greater amount ofreflux will allow a lower vacuum pressure to be attained. Therefore theamount of reflux and the pump P2 should be adjusted to allow achievementof the vacuum levels set forth above, particularly in vessel D2. Thetemperature of the reflux in column 255 will also have an even moresignificant impact on the vacuum pressure by setting a minimum limit onthe vacuum pressure. At a given reflux concentration and pressure, alower temperature for the reflux through column 255 will permit a lowervacuum pressure to be obtained. FIG. 8 shows, for two given pressures (5and 10 torr) in the column 255, what temperature the reflux (of aparticular concentration) should be kept at in order to obtain thedesired vacuum of 5 or 10 torr. For example, if the desired vacuum is 5torr and the dilute acid concentration (by percent weight of H₂ SO₄) is50%, then the dilute acid reflux should be kept at about 15° C. Inpractice, the temperature of the reflux is controlled largely bycontrolling the temperature of the cooling water which flows through thecoil 256 of HE1 or by controlling the size of HE1. The reflux of diluteacid in column 255 serves as a means for setting the minimum vacuumpressure in the distillation system. Alternatively, the reflux of diluteacid through the column 255 may be replaced by a condenser coil locatednear the top of the column 255, which coil acts as a means for settingthe minimum pressure. This condenser coil (through which a coolantflows) may be provided near the top of the column 255 instead of thecolumn packing AD1 and instead of the reflux input 23b. The condensercoil would work in the same fashion as the reflux of dilute acid;specifically, a lower temperature for the condenser coil (at a givenconcentration of dilute acid in column 255) permits a lower (minimum)vacuum pressure to be attained.

C. Quality Assurance

Referring to FIGS. 2 and 7, the in-line quality assurance loop isdescribed. The in-line quality assurance system 7 is the most reliableway to obtain an accurate particle count (BB1), since the sampling istaken directly from the distillation column rather than removing theliquid product from the receiving tank T2 where the possibility ofhandling contamination is increased.

The product received in the pre-product flask F2 is monitored bytemperature sensor T15 and level sensor L5. Once the desired level ofproduct in flask F2 is attained, some of the product is drained throughvalve CV2 and valve V6 into the quality assurance loop. Once asufficient amount of the product is through valve V6, valve V6 is closedand the product is processed in the conventional particle counter BB1, aconventional density monitor BB3 and finally in a conventional metal ionmeasurement device (e.g., ion chromatograph BB2).

If the purity is within the desired specifications valve V18 is closedand valves V5 and CV2 are opened and the product in the pre-productflask F2 drains into the product collection tank T-3 through line 111,valve V7 and line 112.

D. Recycling

If the purity as determined by the quality assurance system 7 is notwithin the desired specifications, the product in pre-product flask F2is recycled back into the feed line 1 through line 111 and line 113 byopening valve V8 while keeping valve V7 closed. This allows the productnot meeting the requisite purity standard to be recycled through thedistillation process.

We claim:
 1. An acid reprocessor for reprocessing waste piranhacontaining contaminated sulfuric acid (H₂ SO₄) from a semiconductorprocessing operation, said acid reprocessor comprising:an input flaskmeans for receiving said waste piranha containing contaminated H₂ SO₄,light boiling contaminants, water and particulates, said input flaskmeans including a first heating means for heating said waste piranha,said input flask means having an output for outputting said wastepiranha; a first distillation means having an input coupled to saidoutput of said input flask means to receive said waste piranha andhaving a gaseous output being coupled to a first column packed with acolumn packing means, said first distillation means having a secondheating means for heating said waste piranha to boil off said water andsaid light boiling contaminants from said waste piranha to produce anenriched acid, said contaminants escaping from said gaseous outputthrough said first column and through a mist eliminator means of saidfirst distillation means, said first column having an input to receivereflux liquid which is trickled through said column to retard loss of H₂SO₄ in said first distillation means, said first distillation meanshaving a feed output for providing said enriched acid; a seconddistillation means having an input coupled to said feed output forreceiving said enriched acid, said second distillation means having athird heating means for heating said enriched acid to boil offsubstantially pure H₂ SO₄, through a second column leaving an acid wastesludge in said second distillation means, said second distillation meanshaving a first output for providing said substantially pure H₂ SO₄, saidfirst output being coupled to a condenser to condense said substantiallypure H₂ SO₄, said condenser having a first coil through which a coolantflows to cool the substantially pure H₂ SO₄ which flows through saidcondenser, said second distillation means having a second output forcontinuously purging said acid waste sludge from said seconddistillation means, said second distillation means having a structurebeing comprised substantially of borosilicate glass; a heat exchangermeans having a cooling means including a second coil through which acoolant flows and having a first input coupled to said gaseous output ofsaid first distillation means to receive said water and said lightboiling contaminants escaping from said first distillation means andhaving a second input coupled to said second output of said seconddistillation means to receive said acid waste sludge, said heatexchanger means having a third column packed with column packing meansand having an input to receive waste acid which is trickled through saidthird column; a vacuum generation means coupled to provide a vacuum insaid first distillation means and in said second distillation means,said vacuum generation means producing a vacuum in said seconddistillation means to reduce said contaminants of said substantiallypure H₂ SO₄, whereby said acid reprocessor provides a reprocessedsemiconductor grade H₂ SO₄ from waste piranha of a semiconductorprocessing operation.
 2. The acid reprocessor as described in claim 1wherein said input to receive waste acid which is trickled through saidthird column provides a means for setting the minimum vacuum pressure inthe distillation system and wherein said first distillation means is astructure being comprised substantially of borosilicate glass.
 3. Anacid reprocessor for reprocessing waste piranha containing contaminatedH₂ SO₄ from a semiconductor processing operation, said acid reprocessorcomprising:an input flask means for receiving said waste piranhacontaining contaminated H₂ SO₄ light boiling contaminants, particulatesand water, said input flash means having an output for outputting saidwaste piranha; a first distillation means having an input coupled tosaid output of said input flask means to receive said waste piranha andhaving a gaseous output being coupled to a first column packed with acolumn packing means, a first heating means for heating said wastepiranha in said first distillation means to boil off water and otherlight boiling contaminants from said waste piranha to produce anenriched acid, said light boiling contaminants and water escaping fromsaid gaseous output through said first column of said first distillationmeans, an input in said first column to receive reflux liquid which istrickled through said first column to retard loss of H₂ SO₄ in saidfirst distillation means, a feed output from said distillation means forsaid enriched acid; a second distillation means having an input coupledto said feed output for receiving said enriched acid, a second heatingmeans for heating said enriched acid in said second distillation meansto boil off substantially pure H₂ SO₄, through a second column leavingan acid waste sludge in said second distillation means, said seconddistillation means having a first output for providing saidsubstantially pure H₂ SO₄, said first output being coupled to acondenser to condense said substantially pure H₂ SO₄, said seconddistillation means having a second output for removing continuously saidacid waste sludge from said second distillation means; a vacuumgeneration means coupled to said first and second distillation means,providing a reduced operating pressure in said first and seconddistillation means, such that said particulates remain in said acidwaste sludge; a heat exchanger means having a third column and a coolingmeans, said heat exchanger means having a first input coupled to saidgaseous output of said first distillation means to receive said waterand said light boiling contaminants from said first distillation meansand having a second input coupled to said second output of said seconddistillation means to receive said acid waste sludge, said cooling meansincluding a first chamber through which a coolant flows to cool saidacid waste sludge, said third column being packed with a column packingmeans and being coupled to said vacuum generation means to provide saidreduced operating pressure in said first and said second distillationmeans, said third column having a third input to receive a waste acidwhich is trickled through said third column, such that said third inputprovides a means for setting the minimum vacuum pressure in thedistillation system and for removing any acid vapor from a vapor streamcreated in said heat exchanger means, said heat exchanger being coupledto said waste collection tank to provide for removal of said acid wastesludge from said second distillation means through said second output ofsaid second distillation means; and a waste collection tank providingfor removal of said acid waste sludge from said second distillationmeans through said second output.
 4. The acid reprocessor as describedin claim 3 wherein said first distillation means, said seconddistillation means and said heat exchanger means are comprised ofsubstantially a borosilicate glass.
 5. An acid reprocessor as in claim 3further comprising a valve means coupled to said second output of saidsecond distillation means to control the rate of removal of said acidwaste sludge from said second distillation means, wherein said valve isperiodically opened and closed to continuously remove said acid wastesludge from said second distillation means.